One conventional instrument for measuring the profile of a surface, to determine surface roughness, comprises a flat annular plate of a particular known size. A resiliently mounted pointed tip protrudes through the central aperture in the plate, and the instrument displays or records the distance which the tip extends beyond the plate.
In use the instrument is placed on a surface to be measured so that it is supported on the plate. The plate will rest on peaks in the surface. The tip is dimensioned so that it can rest in valleys in the surface. Depending on placement of the instrument on a surface the tip will sometimes extend into a valley on the surface, sometimes rest on a peak and sometimes rest at some intermediate position. Multiple measurements are taken placing the instrument in a different position on the surface for each measurement. The range of measurements taken will give an indication of the average profile of the surface over an area.
This type of instrument is intended for measuring the profile of a substantially flat surface. It is not suitable for measuring the profile of a curved surface, because it will not stably rest on such a surface and so accurate measurements cannot be made.
It is an object of embodiments of the present invention to address this problem.